In order to prevent the exhaust of unburned fuel, diesel engines can be run with a lean air-to-fuel ratio (greater than stoichiometric). However, the excess oxygen present within lean combustion produces nitrogen oxides. Therefore, diesel engines produce a significant amount of nitrogen oxides amongst other various pollutants. Government regulations require certain measures to be taken to limit the amount of pollutants that enter the atmosphere. This includes providing exhaust aftertreatment devices, such as selective catalytic reduction (SCR) systems, within the exhaust of many diesel vehicles.
SCR systems contain a catalyst washcoat through which the exhaust of the diesel engine must pass. Periodically, a diesel exhaust fluid or reductant, such as urea, anhydrous ammonia, or aqueous ammonia is dosed in front of the catalyst. This fluid decomposes into ammonia and other gases, and the ammonia is adsorbed in the catalyst washcoat. As the exhaust gas passes through the washcoat containing the ammonia, a reduction reaction converts the pollutants (e.g., NOx) to less harmful products: nitrogen (N2), water (H2O), and/or carbon dioxide (CO2), depending on the diesel exhaust fluid used. The scheduled timing of the dosing, as well as the amount of diesel exhaust fluid dosed is very important to ensure that the SCR system functions correctly. If the catalyst washcoat does not contain enough adsorbed ammonia, pollutants will pass through the SCR system unreacted. If too much diesel exhaust fluid is injected, the ammonia passes through the SCR system unreacted.